1. Field of the Invention
The present invention relates implanted battery operated cardiac pacemakers employing battery energy saving features, and more particularly, it relates to means and methods for controlling one pacemaker sensor by another pacemaker sensor to maximize conservation of battery energy.
2. Description of the Prior Art
Rate responsive pacemaker systems are widely available in the art. Rate responsive systems contain means for monitoring at least one patient variable and for determining an indicated pacing rate as a function of such sensed pacing variable, so as to control pacing rate optimally in terms of the patient condition. Such rate responsive pacemakers have gained wide acceptance as providing an improved response to the patient's physiological needs, as compared to programmable fixed rate pacemakers.
A number of patient variables or rate control parameters have been suggested in the technical literature and used commercially. One physiological parameter utilized for rate control is patient activity level. Activity sensors have been widely utilized for detecting the general activity level of a patient with a pacemaker, and for controlling the pacing rate or escape interval in response to detected activity level such as that disclosed in U.S. Pat. No. 4,428,378 issued to Anderson et al. and assigned to the assignee of the present invention and which is incorporated herein by reference in its entirety.
Recent approaches to optimizing rate responsiveness use dual or plural sensors, in order that the drawbacks or deficiencies of a given sensor and/or algorithm may be compensated by the use of a second or other sensors having different characteristics. This approach is set forth in U.S. Pat. No. 4,527,568 issued to Rickards, which discloses switching control of rate responsiveness from one monitored parameter to another control parameter. There are many other examples of multiple sensor approaches in the literature, and reference is made to U.S. Pat. Nos. 5,101,824, 4,926,863 and 4,905,697. These references are characterized by designs which switch control from one sensor to another, or from one algorithm to another, depending upon monitored values of the rate control parameters.
Many rate responsive, multiple sensor cardiac pacemakers, including some of those discussed above, achieve a reduction in energy consumption in order to improve battery longevity, through the use of rate optimization. In addition, energy saving circuitry measures have reduced the pacemaker's power consumption. The development of modern electrodes that build up very small polarization voltages has also contributed to reducing the energy required for pacing. In U.S. Pat. No. 4,979,507 to Heinz et al., an energy saving cardiac pacemaker is disclosed which is based on optimizing stimulus thresholds. However, none of the above multiple sensor cardiac pacemaker art discloses controlling one physiological sensor with another physiological sensor for the purpose of minimizing battery energy consumption without impairment of the blending of multiple sensor outputs to produce a composite optimal heart rate.